Re: [PATCH] cgroup: memory.force_empty can make system slowdown

[Li Zefan <lizf@cn.fujitsu.com>]

Li Zefan wrote:
> IKEDA, Munehiro wrote:
>> Cgroup's memory controller has a control file "memory.force_empty"
>> to reset usage account charged to a cgroup.  The account shouldn't
>> be reset if one or more processes are attached to the cgroup (at
>> least for memory controller, IMHO).  So mem_cgroup_force_empty()
>> is implemented to return -EBUSY and do nothing if so.
>> However, cgroup on hierarchy root faultily might be a exception.
>> Even if processes are attached to root cgroup (which is a "default"
>> cgroup for processes), forcing-empty can run by writing something to
>> memory.force_empty and it'll never end.
>>
> 
> I found this bug last week, and I've made patches to fix it, but then
> I was on vacation. I'll send the patches out soon.
> 
>> Following patch prevents this issue.
>>
>> This patch is for cgroup infrastructure code.  The issue can be
>> measured by modifying memory controller code also, namely to change
>> mem_cgroup_force_empty() to see CSS_ROOT bit of css->flags.
>> I believe cgroup->count approach like the patch below is rather
>> generic and reasonable, how does that sound?
>>
> 
> It's ok for the top_group's count to be 0 due to the top_cgroup hack.
> With this patch, the top cgroup's count will be always >0, even if it
> has no tasks in it, so writing to top_cgroup's force_empty will always
> return -EBUSY.
> 

I thought cgrp->css_sets will be empty when there are no tasks in the top cgroup,
but I was wrong, because init_css_set's refcount will always >0,
so cgroup_task_count() won't return 0 for the top cgroup:

# mount -t cgroup -o debug xxx /mnt
# mkdir /mnt/sub
# for pid in `cat /mnt/tasks`; do echo $pid > /mnt/sub/tasks; done
# cat /mnt/tasks
# cat /mnt/debug.taskcount
3